In assembling miniature components, for example in electronics work, the art of bonding such small components is a challenging task. Bonding methods and means have been developed for conventional semiconductor work, such as in bonding leads to pads on a chip. However, there seems to be a paucity of art available concerning the assembly of lightwave devices; yet such assembly is emerging as a relevant and material part of semiconductor work.
For example, in processing light from a glass fiber, a semiconductor device such as an avalanche photodiode (APD) formed in silicon is often utilized to convert such light to electrical pulses. To couple output light from a fiber to an input junction of an APD, a guide is conveniently employed having a body typically about 1/8-inch in diameter and length. A disc-like stud having an APD mounted centrally thereon is bonded at one end of the guide with the APD facing a bore in the body. When the APD is properly positioned and the guide is slipped over the end of a fiber, light emitted into the bore impinges upon and operates the APD. A problem is that the stud is so small it is difficult to bond to the body with the APD in precise registration with the bore. Another problem is that the APD and its chip mounting to the stud are undesirably sensitive to high temperatures sometimes experienced in thermal bonding.
To register the APD, the chip is mounted to a pedestal which is located centrally of the stud. Accordingly, only an annular ring surface of the stud makes contact, ideally with a corresponding metallized ring seat on the body of the guide. A ring member of preformed solder resembling a washer is installed therebetween which melts on heating and solidifies on cooling to bond a ring portion of the stud to the seat. A problem is to retain the stud and the washer in registration with the seat to make a sufficiently strong joint that the assembly may be readily removed and a new work cycle initiated.
In a conventional method of heating small workpieces, a heater having the shape of a rifle cartridge is often utilized. The heater has a thin axial member made of a refractory material around which there is wound a fine heating wire. Then an insulative sleeve encloses the axial member and wire to avoid grounding and a metal enclosure with electrical connections completes the unit. Such a heater is normally inserted vertically into and flush with a work surface whereon assemblies are made and a workpiece is positioned on top of the cartridge heater as one would dispose a pot on an electric stove. A problem with this method is that it takes an unacceptably long time to heat the cartridge and the workpiece thereon sufficiently to liquify a solder washer. Another problem is that the large mass of the heater retains heat unacceptably long after electrical current ceases. Consequently, time is required to cool the workpiece sufficiently so an assembly may be readily removed without breaking or otherwise disturbing a bonded joint.
Accordingly, it is desirable to develop new and improved expedients for thermally bonding a workpiece especially a minute workpiece, to a small body. It is further desirable to bond workpieces which have discrete bonding surfaces distinct from other portions of a workpiece. In particular, the heat utilized should be rapidly developed and rapidly dissipated to expedite work cycles in assembling components. Moreover, it is desirable to develop heat in a workpiece primarily at portions requiring heat for bonding and substantially avoiding heat at other portions which may be undesirably heat sensitive.